Semiconductor device and method for fabricating the same

ABSTRACT

An impurity diffusion layer serving as the source or the drain of a transistor is formed in a semiconductor substrate, and a protection insulating film is formed so as to cover the transistor. A capacitor lower electrode, a capacitor dielectric film of an oxide dielectric film and a capacitor upper electrode are successively formed on the protection insulating film. A plug for electrically connecting the impurity diffusion layer of the transistor to the capacitor lower electrode is buried in the protection insulating film. An oxygen barrier layer is formed between the plug and the capacitor lower electrode. The oxygen barrier layer is made from a composite nitride that is a mixture or an alloy of a first nitride having a conducting property and a second nitride having an insulating property.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a semiconductor device includinga capacitor device having a capacitor dielectric film of an oxidedielectric film such as a ferroelectric film and a high dielectric film,and a method for fabricating the semiconductor device.

[0002] In a recently accelerated trend in processing and storing massivedata resulting from development of digital technology, electronicequipment have been more and more highly developed, and therefore,semiconductor devices used in electronic equipment have been rapidlydeveloped in their refinement.

[0003] Accordingly, in order to realize a high degree of integration ina dynamic RAM, a technique to use an oxide dielectric film as acapacitor dielectric film instead of a conventionally used silicon oxideor silicon nitride film has been widely studied and developed.

[0004] Also, in order to realize practical use of a nonvolatile RAMcapable of operating at a lower voltage and writing/reading data at ahigher speed, ferroelectric films having a spontaneous polarizationcharacteristic are earnestly studied.

[0005] In a semiconductor memory using a ferroelectric film or a highdielectric film, in order to attain a high degree of integration of amegabit-class, stack-type memory cells are used instead ofconventionally used planer-type memory cells. The most significantproblem in employing the stack-type memory cells is preventing a contactface between a plug and a lower electrode of a capacitor device frombeing oxidized in high temperature annealing carried out in an oxygenatmosphere for crystallizing the ferroelectric film or the highdielectric film.

[0006] A conventional semiconductor device will now be described withreference to FIG. 6A.

[0007] As shown in FIG. 6A, impurity diffusion layers 11 serving as thesource and the drain are formed in a semiconductor substrate 10, and agate electrode 12 is formed on a region of the semiconductor substrate10 sandwiched between the impurity diffusion layers 11. The impuritydiffusion layers 11 and the gate electrode 12 together form atransistor.

[0008] A protection insulating film 13 is formed on the semiconductorsubstrate 10 so as to cover the transistor, and a plug 14 of, forexample, tungsten connected to one of the impurity diffusion layers 11is formed in the protection insulating film 13.

[0009] An adhesive layer 15 of titanium having a lower face in contactwith the upper face of the plug 14 is formed on the protectioninsulating film 13. An oxygen barrier layer 16 of iridium oxide isformed on the adhesive layer 15, and a capacitor device composed of acapacitor lower electrode 17, a capacitor dielectric film 18 of aferroelectric film and a capacitor upper electrode 19 is formed on theoxygen barrier layer 16. Accordingly, one of the impurity diffusionlayers 11 of the transistor is electrically connected to the capacitorlower electrode 17 through the plug 14.

[0010] The oxygen barrier layer 16 has a function to prevent oxidationof the plug 14, and the adhesive layer 15 has a function to improveadhesion between the oxygen barrier layer 16 and the plug 14.

[0011] In order to crystallize the ferroelectric film used for formingthe capacitor dielectric film 18, it is necessary to carry out annealingat a temperature of 600 through 800 in an oxygen atmosphere. During thisannealing, a metal oxide film with high resistance is formed in thevicinity of the interface between the plug 14 and the adhesive layer 15,which disadvantageously increases the contact resistance between theplug 14 and the lower electrode 17.

[0012] Therefore, the present inventors have variously studied the causeof the formation of the metal oxide film in the vicinity of theinterface between the plug 14 and the adhesive layer 15, resulting infinding the following:

[0013]FIG. 6B shows migration paths of oxygen atoms in the conventionalsemiconductor device, wherein denotes an oxygen atom and an arrowdenotes a migration path of the oxygen atom.

[0014] In the annealing for crystallizing the ferroelectric film usedfor forming the capacitor dielectric film 18, oxygen atoms included inthe oxygen atmosphere are diffused into the capacitor dielectric film18, then migrate through a first path for passing through the capacitorlower electrode 17 and the oxygen barrier layer 16 to reach the adhesivelayer 15 and through a second path for passing through a side portion ofthe capacitor dielectric film 18 to reach the adhesive layer 15, andfinally reach the plug 14.

[0015] Although the oxygen barrier layer 16 of iridium oxide is formedon the plug 14, the oxygen barrier layer 16 cannot definitely preventthe passage of the oxygen atoms because the annealing forcrystallization is carried out in an oxygen atmosphere at a hightemperature.

[0016] Also, when the oxygen atoms reach the adhesive layer 15, titaniumincluded in the adhesive layer 15 is easily oxidized into titaniumoxide, and hence, the oxygen atoms reach the plug 14 after thusoxidizing the adhesive layer. The oxygen atoms having reached the plug14 oxidize a metal, such as tungsten, included in the plug 14, whichdisadvantageously increases the contact resistance between the capacitorlower electrode 17 and the plug 14.

[0017] Furthermore, when the oxygen atoms reach the oxygen barrier layer16, pin holes may be formed or the thickness is locally reduced in theoxygen barrier layer 16. Therefore, in a contact chain used for a testand including thousands or ten thousands of serially connected plugs 14,the resistance becomes abnormally high when the diameter of each plug 14is small.

SUMMARY OF THE INVENTION

[0018] In consideration of the aforementioned conventional problems, anobject of the invention is preventing contact resistance between acapacitor lower electrode and a plug from increasing by definitelypreventing oxidation of the plug.

[0019] In order to achieve the object, the first semiconductor device ofthis invention comprises an impurity diffusion layer serving as a sourceor a drain of a transistor formed in a semiconductor substrate; aprotection insulating film covering the transistor; a capacitor lowerelectrode, a capacitor dielectric film of an oxide dielectric film and acapacitor upper electrode successively formed on the protectioninsulating film; a plug buried in the protection insulating film forelectrically connecting the impurity diffusion layer of the transistorto the capacitor lower electrode; and an oxygen barrier layer formedbetween the plug and the capacitor lower electrode, and the oxygenbarrier layer is made from a composite nitride that is a mixture or analloy of a first nitride having a conducting property and a secondnitride having an insulating property.

[0020] In the first semiconductor device of the invention, the oxygenbarrier layer formed between the plug and the capacitor lower electrodeis made from the composite nitride that is a mixture or an alloy of thefirst nitride having a conducting property and the second nitride havingan insulating property. In an oxygen atmosphere at a high temperature,the second nitride having an insulating property is more highly reactivewith oxygen atoms than the first nitride having a conducting property.

[0021] Therefore, in crystallizing the capacitor dielectric film of theoxide dielectric film in an oxygen atmosphere at a high temperature,when the oxygen atoms diffuse into the oxygen barrier layer, the secondnitride having an insulating property is rapidly reacted with the oxygenatoms to produce an oxide in a surface portion of the oxygen barrierlayer. Since an oxide has a smaller particle size than a nitride, whenthe nitride is changed into the oxide, the migration paths of the oxygenatoms formed in the grain boundary of the nitride becomes complicatedand elongated, which makes it difficult for the oxygen atoms to diffusewithin the oxygen barrier layer. In other words, since an oxide layerfor preventing diffusion of the oxygen atoms is formed in the surfaceportion of the oxygen barrier layer, the function of the oxygen barrierlayer to prevent diffusion of the oxygen atoms can be improved.

[0022] When the nitride is changed into the oxide, the resistance of theoxygen barrier layer may be increased. The composite nitride includes,however, the first nitride having a conducting property that iscomparatively less reactive with the oxygen atoms, which suppresses theincrease of the resistance of the oxygen barrier layer.

[0023] Accordingly, while suppressing the increase of the resistance,the oxygen barrier layer can definitely prevent the diffusion of theoxygen atoms, resulting in definitely preventing oxidation of the plug.

[0024] In the first semiconductor device, the first nitride ispreferably a nitride of at least one of titanium, tantalum, cobalt,copper and gallium, and the second nitride is preferably a nitride of atleast one of aluminum, silicon, chromium, iron, zirconium and hafnium.

[0025] When the nitride of aluminum, silicon, chromium, iron, zirconiumor hafnium is brought into contact with oxygen atoms at a hightemperature, the nitride rapidly changes into an oxide, so as to preventthe diffusion of the oxygen atoms. Therefore, the function of the oxygenbarrier layer to prevent the diffusion of the oxygen atoms can bedefinitely improved. Furthermore, since the nitride of titanium,tantalum, cobalt, copper or gallium is difficult to oxidize even at ahigh temperature and is less degraded in its conducting property evenwhen oxidized, the increase of the resistance of the oxygen barrierlayer can be suppressed.

[0026] The first semiconductor device preferably further comprises anupper oxygen barrier layer formed between the oxygen barrier layer andthe capacitor lower electrode and made from a metal that has aconducting property when it is oxidized.

[0027] When the upper oxygen barrier layer of the metal that has aconducting property even when oxidized is thus formed on the oxygenbarrier layer, two oxygen barrier layers are present on the plug.Therefore, the function to prevent the diffusion of the oxygen atoms canbe further improved and the increase of the resistance can be prevented.

[0028] In this case, the metal is preferably at least one of iridium,ruthenium, rhenium, osmium, rhodium, platinum and gold.

[0029] Thus, when the oxygen atoms diffuse into the upper oxygen barrierlayer, a metal oxide layer that prevents the migration of the oxygenatoms and does not have very high resistance is formed in a surfaceportion of the upper oxygen barrier layer. Therefore, the diffusion ofthe oxygen atoms can be more effectively prevented.

[0030] The first semiconductor device preferably further comprises anupper oxygen barrier layer formed between the oxygen barrier layer andthe capacitor lower electrode and made from a metal oxide having aconducting property.

[0031] When the upper oxygen barrier layer of the metal oxide having aconducting property is thus formed on the oxygen barrier layer, twooxygen barrier layers are present on the plug. Therefore, the functionto prevent the diffusion of the oxygen atoms can be further improved andthe increase of the resistance can be prevented.

[0032] In this case, the metal oxide is preferably at least one of aniridium oxide, a ruthenium oxide, a rhenium oxide, an osmium oxide and arhodium oxide.

[0033] Thus, the oxygen atoms diffusing through the upper oxygen barrierlayer are prevented from migrating by the metal oxide, and hence, thediffusion of the oxygen atoms can be more effectively prevented.

[0034] The first semiconductor device preferably further comprises anupper oxygen barrier layer of a multi-layer structure composed of afirst metal layer of a metal that has a conducting property when it isoxidized and a second metal layer of a metal oxide having a conductingproperty.

[0035] Thus, even when a defect is caused in one of the first metallayer and the second metal layer, the other metal layer can prevent thepassage of the oxygen atoms. Therefore, the diffusion of the oxygenatoms can be definitely prevented.

[0036] The second semiconductor device of this invention comprises animpurity diffusion layer serving as a source or a drain of a transistorformed in a semiconductor substrate; a first protection insulating filmcovering the transistor; a plug buried in the first protectioninsulating film and having a lower end electrically connected to theimpurity diffusion layer of the transistor; an oxygen barrier layerformed on the first protection insulating film and having a lower faceconnected to an upper end of the plug; a capacitor lower electrodeformed on the oxygen barrier layer; a second protection insulating filmformed on the first protection insulating film to cover peripheral facesof the oxygen barrier layer and the capacitor lower electrode and havingan upper face placed at substantially the same level as an upper face ofthe capacitor lower electrode; a capacitor dielectric film made from anoxide dielectric film formed on the capacitor lower electrode and thesecond protection insulating film and having a plane shape larger than aplane shape of the capacitor lower electrode; and a capacitor upperelectrode formed on the capacitor dielectric film.

[0037] In the second semiconductor device of this invention, the secondprotection insulating film is formed so as to cover the peripheral faceof the oxygen barrier layer. Therefore, in crystallizing the capacitordielectric film of the oxide dielectric film in an oxygen atmosphere ata high temperature, oxygen atoms included in the oxygen atmosphere passthrough the second protection insulating film before reaching the oxygenbarrier layer, and hence, the number of oxygen atoms that can reach theoxygen barrier layer can be reduced. Also, since the capacitordielectric film has a plane shape larger than that of the capacitorlower electrode, the oxygen atoms included in the oxygen atmospheremigrate by a long distance, namely, take a roundabout way, within thesecond protection insulating film before reaching the oxygen barrierlayer. Therefore, the number of oxygen atoms that can reach the oxygenbarrier layer can be further reduced.

[0038] Accordingly, the number of oxygen atoms that diffuse through theoxygen barrier layer to reach the plug can be largely reduced, resultingin definitely preventing the oxidation of the plug.

[0039] In the second semiconductor device, the oxygen barrier layer ispreferably made from a composite nitride that is a mixture or an alloyof a first nitride having a conducting property and a second nitridehaving an insulating property.

[0040] Thus, when the oxygen atoms pass through the second protectioninsulating film to reach the oxygen barrier layer, the second nitridehaving an insulating property is changed into an oxide in a surfaceportion of the oxygen barrier layer. Therefore, the oxygen atoms aredifficult to diffuse into the oxygen barrier layer, resulting in largelyimproving the function of the oxygen barrier layer to prevent thediffusion of the oxygen atoms.

[0041] The second semiconductor device preferably further comprises anupper oxygen barrier layer formed between the oxygen barrier layer andthe capacitor lower electrode and made from a metal that has aconducting property when it is oxidized.

[0042] Thus, two oxygen barrier layers are present on the plug, andhence, the function to prevent the diffusion of the oxygen atoms can befurther improved, and the increase of the resistance can be prevented.

[0043] The second semiconductor device preferably further comprises anupper oxygen barrier layer formed between the oxygen barrier layer andthe capacitor lower electrode and made from a metal oxide having aconducting property.

[0044] Thus, two oxygen barrier layers are present on the plug, andhence, the function to prevent the diffusion of the oxygen atoms can befurther improved, and the increase of the resistance can be prevented.

[0045] The method for fabricating a semiconductor device of thisinvention comprises the steps of forming an impurity diffusion layerserving as a source or a drain of a transistor in a semiconductorsubstrate; forming a first protection insulating film covering thetransistor; burying, in the first protection insulating film, a plughaving a lower end electrically connected to the impurity diffusionlayer of the transistor; forming, on the first protection insulatingfilm, an oxygen barrier layer having a lower face connected to an upperend of the plug; forming a capacitor lower electrode on the oxygenbarrier layer; forming, on the first protection insulating film, asecond protection insulating film covering the oxygen barrier layer andthe capacitor lower electrode, and planarizing the second protectioninsulating film, whereby placing an upper face of the second protectioninsulating film at substantially the same level as an upper face of thecapacitor lower electrode; forming a capacitor dielectric film having aplane shape larger than a plane shape of the capacitor lower electrodeby depositing an oxide dielectric film on the capacitor lower electrodeand the second protection insulating film and patterning the oxidedielectric film; and forming a capacitor upper electrode on thecapacitor dielectric film.

[0046] In the method for fabricating a semiconductor device of thisinvention, after forming the second protection insulating film so as tocover the oxygen barrier layer and the capacitor lower electrode, thesecond protection insulating film is planarized so as to place the upperface of the second protection insulating film at substantially the samelevel as the upper face of the capacitor lower electrode. Therefore, thecapacitor dielectric film of the oxide dielectric film is crystallizedin an oxygen atmosphere at a high temperature with the peripheral faceof the oxygen barrier layer covered with the second protectioninsulating film, and hence, oxygen atoms included in the oxygenatmosphere pass through the second protection insulating film beforereaching the oxygen barrier layer. Also, since the capacitor dielectricfilm has a plane shape larger than that of the capacitor lowerelectrode, the oxygen atoms included in the oxygen atmosphere migrate bya long distance within the second protection insulating film beforereaching the oxygen barrier layer, and hence, the number of oxygen atomsthat can reach the oxygen barrier layer can be largely reduced.

[0047] Accordingly, the number of oxygen atoms that diffuse through theoxygen barrier layer to reach the plug can be largely reduced, resultingin definitely preventing oxidation of the plug.

[0048] In the method for fabricating a semiconductor device, the oxygenbarrier layer is preferably made from a composite nitride that is amixture or an alloy of a first nitride having a conducting property anda second nitride having an insulating property.

[0049] Thus, when the oxygen atoms pass through the second protectioninsulating film to reach the oxygen barrier layer, the second nitridehaving an insulating property is changed into an oxide in a surfaceportion of the oxygen barrier layer. Therefore, the oxygen atoms aredifficult to diffuse into the oxygen barrier layer, and hence, thefunction of the oxygen barrier layer to prevent the diffusion of theoxygen atoms can be largely improved.

[0050] The method for fabricating a semiconductor device preferablyfurther comprises, between the step of forming the oxygen barrier layerand the step of forming the capacitor lower electrode, a step of formingan upper oxygen barrier layer made from a metal that has a conductingproperty when it is oxidized.

[0051] Thus, since two oxygen barrier layers are present on the plug,the function to prevent the diffusion of the oxygen atoms can be furtherimproved, and increase of the resistance can be prevented.

[0052] The method for fabricating a semiconductor device preferablyfurther comprises, between the step of forming the oxygen barrier layerand the step of forming the capacitor lower electrode, a step of formingan upper oxygen barrier layer made from a metal oxide having aconducting property.

[0053] Thus, two oxygen barrier layers are present on the plug, thefunction to prevent the diffusion of the oxygen atoms can be furtherimproved, and the increase of the resistance can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

[0054]FIG. 1A is a cross-sectional view of a semiconductor deviceaccording to Embodiment 1 of the invention, FIG. 1B is a cross-sectionalview of a semiconductor device according to Embodiment 2 of theinvention and FIG. 1C is a cross-sectional view of a semiconductordevice according to Embodiment 3 of the invention;

[0055]FIGS. 2A, 2B and 2C are cross-sectional views for showingprocedures in a method for fabricating the semiconductor device ofEmbodiment 2;

[0056]FIGS. 3A, 3B and 3C are cross-sectional views for showing otherprocedures in the method for fabricating the semiconductor device ofEmbodiment 2;

[0057]FIG. 4 is a diagram for showing the result of measurement of afailure occurrence probability in a contact chain including seriallyconnected 1000 semiconductor devices;

[0058]FIG. 5 is a diagram for showing contact resistance correspondingto resistance of each plug; and

[0059]FIG. 6A is a cross-sectional view of a conventional semiconductordevice and FIG. 6B is a cross-sectional view for explaining a problem ofthe conventional semiconductor device.

DETAILED DESCRIPTION OF THE INVENTION

[0060] Embodiment 1

[0061] A semiconductor device according to Embodiment 1 of the inventionwill now be described with reference to FIG. 1A.

[0062] As shown in FIG. 1A, a pair of impurity diffusion layers 101serving as the source and the drain of a transistor are formed in asemiconductor substrate 100, and a gate electrode 102 of the transistoris formed on a region of the semiconductor substrate 100 sandwichedbetween the pair of impurity diffusion layers 101.

[0063] A first protection insulating film 103 of, for example, a TEOS-O₃film is formed on the semiconductor substrate 100 so as to cover thetransistor, and a plug 107 of tungsten having a lower end connected toone of the pair of impurity diffusion layers 101 is buried in the firstprotection insulating film 103. The plug 107 includes a barrier metalcomposed of an outside titanium film with a thickness of, for example,30 nm and an inside titanium nitride film with a thickness of, forexample, 50 nm. The upper face of the plug 107 is placed atsubstantially the same level as the upper face of the first protectioninsulating film 103.

[0064] An oxygen barrier layer 108A with a thickness of 20 nm through200 nm having a lower face connected to the upper end of the plug 107 isformed on the first protection insulating film 103. The oxygen barrierlayer 108A is made from a composite nitride that is a mixture or analloy of a first nitride having a conducting property and a secondnitride having an insulating property. The first nitride may be anitride of at least one of titanium, tantalum, cobalt, copper andgallium, and the second nitride may be a nitride of at least one ofaluminum, silicon, chromium, iron, zirconium and hafnium.

[0065] A capacitor lower electrode 110A of a platinum film with athickness of, for example, 50 nm is formed on the oxygen barrier layer108A.

[0066] The peripheral faces of the oxygen barrier layer 108A and thecapacitor lower electrode 110A are covered with a second protectioninsulating film 111, and the upper face of the second protectioninsulating film 111 is placed at substantially the same level as theupper face of the capacitor lower electrode 110A.

[0067] A capacitor dielectric film 112A of an oxide dielectric film suchas a ferroelectric film and a high dielectric film with a thickness of10 nm through 200 nm is formed on the second protection insulating film111 so as to have a lower face in contact with the capacitor lowerelectrode 110A. The capacitor dielectric film 112A is in contact withthe capacitor lower electrode 110A and has a plane shape larger thanthat of the capacitor lower electrode 110A. The oxide dielectric film isnot specified in its kind, and may be a ferroelectric film having abismuth-layered perovskite structure such as SrBi₂(Ta_(1-x)Nb_(x))O₉, ora film of lead zirconate titanate, strontium barium titanate, tantalumpentaoxide or the like.

[0068] A capacitor upper electrode 113A of a platinum film with athickness of, for example, approximately 50 nm is formed on thecapacitor dielectric film 112A, and the capacitor dielectric film 112Aand the capacitor upper electrode 113A are covered with a thirdprotection insulating film not shown.

[0069] In the semiconductor device of Embodiment 1, the oxygen barrierlayer 108A of the composite nitride that is a mixture or an alloy of thefirst nitride having a conducting property and the second nitride havingan insulating property is formed between the plug 107 and the capacitorlower electrode 110A. In an oxygen atmosphere at a high temperature, thesecond nitride having an insulating property is more highly reactivewith oxygen atoms than the first nitride having a conducting property.Therefore, in crystallizing the capacitor dielectric film 112A of theoxide dielectric film in an oxygen atmosphere at a high temperature,when oxygen atoms are diffused into the oxygen barrier layer 108A, thesecond nitride having an insulating property is rapidly reacted with theoxygen atoms to produce an oxide in a surface portion of the oxygenbarrier layer 108A. Since an oxide has a smaller particle size than anitride, when the nitride is changed into the oxide, migration paths ofthe oxygen atoms formed in the grain boundary of the nitride becomecomplicated and elongated, which makes difficult for the oxygen atoms todiffuse into the oxygen barrier layer 108A. In other words, an oxidelayer for preventing the diffusion of the oxygen atoms is formed in thesurface portion of the oxygen barrier layer 108A, and hence, thefunction of the oxygen barrier layer 108A for preventing the diffusionof the oxygen atoms can be improved.

[0070] When the nitride is changed into the oxide, the resistance of theoxygen barrier layer 108A may be increased. However, since the compositenitride includes the first nitride having a conducting property that iscomparatively less reactive with the oxygen atoms, the increase of theresistance of the oxygen barrier layer 108A can be suppressed.

[0071] Accordingly, while suppressing the increase of the resistance,the oxygen barrier layer 108A can definitely prevent the diffusion ofthe oxygen atoms, and therefore, oxidation of the plug 107 can bedefinitely prevented.

[0072] Furthermore, since the second nitride included in the oxygenbarrier layer 108A is made from a nitride of aluminum, silicon,chromium, iron, zirconium or hafnium, when it is brought into contactwith the oxygen atoms at a high temperature, it is rapidly changed intothe oxide, resulting in preventing the diffusion of the oxygen atoms.Accordingly, the function of the oxygen barrier layer 108A to preventthe diffusion of the oxygen atoms can be definitely improved.

[0073] Moreover, since the first nitride included in the oxygen barrierlayer 108A is made from a nitride of titanium, tantalum, cobalt, copperor gallium, it is difficult to oxide even at a high temperature, andeven when it is oxidized, its conducting property is less degraded.Therefore, the increase of the resistance of the oxygen barrier layer108A can be suppressed.

[0074] Also, since the peripheral face of the oxygen barrier layer 108Ais covered with the second protection insulating film 111, incrystallizing the capacitor dielectric film 112A of the oxide dielectricfilm in an oxygen atmosphere at a high temperature, the oxygen atomsincluded in the oxygen atmosphere pass through the second protectioninsulating film 111 before reaching the oxygen barrier layer 108A.Therefore, the number of oxygen atoms that can reach the oxygen barrierlayer 108A can be reduced.

[0075] In addition, since the capacitor dielectric film 112A has a planeshape larger than that of the capacitor lower electrode 110A, the oxygenatoms included in the oxygen atmosphere migrate by a long distance,namely, take a roundabout way, within the second protection insulatingfilm 111 before reaching the oxygen barrier layer 108A. Therefore, thenumber of oxygen atoms that can reach the oxygen barrier layer 108A canbe further reduced.

[0076] Accordingly, since the number of oxygen atoms that are diffusedthrough the oxygen barrier layer 108A to reach the plug 107 can belargely reduced, the oxidation of the plug 107 can be definitelyprevented.

[0077] Embodiment 2

[0078] A semiconductor device according to Embodiment 2 of the inventionwill now be described with reference to FIG. 1B.

[0079] As shown in FIG. 1B, a pair of impurity diffusion layers 101serving as the source and the drain of a transistor are formed in asemiconductor substrate 100, and a gate electrode 102 of the transistoris formed on a region of the semiconductor substrate 100 sandwichedbetween the pair of impurity diffusion layers 101.

[0080] A first protection insulating film 103 of, for example, a TEOS-O₃film is formed on the semiconductor substrate 100 so as to cover thetransistor, and a plug 107 of tungsten having a lower end connected toone of the pair of impurity diffusion layers 101 is buried in the firstprotection insulating film 103. The plug 107 includes a barrier metalcomposed of, for example, a titanium film and a titanium nitride film,and the upper face of the plug 107 is placed at substantially the samelevel as the upper face of the first protection insulating film 103.

[0081] An oxygen barrier layer 108A with a thickness of 20 nm through200 nm having a lower face connected to the upper end of the plug 107 isformed on the first protection insulating film 103. The oxygen barrierlayer 108A is made from a composite nitride that is a mixture or analloy of a first nitride having a conducting property and a secondnitride having an insulating property. The first nitride may be anitride of at least one of titanium, tantalum, cobalt, copper andgallium, and the second nitride may be a nitride of at least one ofaluminum, silicon, chromium, iron, zirconium and hafnium.

[0082] An upper oxygen barrier layer 109A with a thickness of, forexample, 100 nm of a metal that has a conducting property even whenoxidized is formed on the oxygen barrier layer 108A. The metal that hasa conducting property even when oxidized may be at least one of iridium,ruthenium, rhenium, osmium, rhodium, platinum and gold.

[0083] A capacitor lower electrode 110A of a platinum film with athickness of, for example, 50 nm is formed on the upper oxygen barrierlayer 109A. The peripheral faces of the oxygen barrier layer 108A, theupper oxygen barrier layer 109A and the capacitor lower electrode 110Aare covered with a second protection insulating film 111, and the upperface of the second protection insulating film 111 is placed atsubstantially the same level as the upper face of the capacitor lowerelectrode 110A.

[0084] A capacitor dielectric film 112A of an oxide dielectric film suchas a ferroelectric film and a high dielectric film with a thickness of10 nm through 200 nm is formed on the second protection insulating film111 so as to have a lower face in contact with the capacitor lowerelectrode 110A. The capacitor dielectric film 112A is in contact withthe capacitor lower electrode 110A and has a plane shape larger thanthat of the capacitor lower electrode 110A. The oxide dielectric film isnot specified in its kind, and may be a ferroelectric film having abismuth-layered perovskite structure such as SrBi₂(Ta_(1-x)Nb_(x))O₉, ora film of lead zirconate titanate, strontium barium titanate, tantalumpentaoxide or the like.

[0085] A capacitor upper electrode 113A of a platinum film with athickness of, for example, approximately 50 nm is formed on thecapacitor dielectric film 112A, and the capacitor dielectric film 112Aand the capacitor upper electrode 113A are covered with a thirdprotection insulating film not shown.

[0086] In the semiconductor device of Embodiment 2, the upper oxygenbarrier layer 109A is formed between the oxygen barrier layer 108A andthe capacitor lower electrode 110A. Therefore, the diffusion of theoxygen atoms can be more effectively prevented than in the semiconductordevice of Embodiment 1, and hence, the oxidation of the plug 107 can bemore definitely prevented.

[0087] In the case where the upper oxygen barrier layer 109A is madefrom at least one of iridium, ruthenium, rhenium, osmium, rhodium,platinum and gold, a metal oxide layer that prevents the migration ofthe oxygen atoms and does not largely increase the resistance is formedin a surface portion of the upper oxygen barrier layer 109A when theoxygen atoms are diffused into the upper oxygen barrier layer 109A.Accordingly, the diffusion of the oxygen atoms can be more definitelyprevented.

[0088] Instead of the metal that has a conducting property even whenoxidized, a metal oxide having a conducting property including at leastone of an iridium oxide, a ruthenium oxide, a rhenium oxide, an osmiumoxide and a rhodium oxide may be used as the metal included in the upperoxygen barrier layer 109A.

[0089] Embodiment 3

[0090] A semiconductor device according to Embodiment 3 of the inventionwill now be described with reference to FIG. 1C.

[0091] As shown in FIG. 1C, a pair of impurity diffusion layers 101serving as the source and the drain of a transistor are formed in asemiconductor substrate 100, and a gate electrode 102 of the transistoris formed on a region of the semiconductor substrate 100 sandwichedbetween the pair of impurity diffusion layers 101.

[0092] A first protection insulating film 103 of, for example, a TEOS-O₃film is formed on the semiconductor substrate 100 so as to cover thetransistor, and a plug 107 of tungsten having a lower end connected toone of the pair of impurity diffusion layers 101 is buried in the firstprotection insulating film 103. The plug 107 includes a barrier metalcomposed of, for example, a titanium film and a titanium nitride film.The upper face of the plug 107 is placed at substantially the same levelas the upper face of the first protection insulating film 103.

[0093] An oxygen barrier layer 108A with a thickness of 20 nm through200 nm having a lower face connected to the upper end of the plug 107 isformed on the first protection insulating film 103. The oxygen barrierlayer 108A is made from a composite nitride that is a mixture or analloy of a first nitride having a conducting property and a secondnitride having an insulating property. The first nitride may be anitride of at least one of titanium, tantalum, cobalt, copper andgallium, and the second nitride may be a nitride of at least one ofaluminum, silicon, chromium, iron, zirconium and hafnium.

[0094] A first upper oxygen barrier layer 114A of a metal that has aconducting property even when oxidized and a second upper oxygen barrierlayer 115A of a metal oxide having a conducting property aresuccessively formed on the oxygen barrier layer 108A. Either of thefirst upper oxygen barrier layer 114A and the second upper oxygenbarrier layer 115A may be disposed below. The metal that has aconducting property even when oxidized used for forming the first upperoxygen barrier layer 114A may be at least one of iridium, ruthenium,rhenium, osmium, rhodium, platinum and gold. The metal oxide having aconducting property used for forming the second upper oxygen barrierlayer 115A may be at least one of an iridium oxide, a ruthenium oxide, arhenium oxide, an osmium oxide and a rhodium oxide.

[0095] A capacitor lower electrode 110A of a platinum film with athickness of, for example, 50 nm is formed on the second upper oxygenbarrier layer 115A. The peripheral faces of the oxygen barrier layer108A, the first upper oxygen barrier layer 114A, the second upper oxygenbarrier layer 115A and the capacitor lower electrode 110A are coveredwith a second protection insulating film 111, and the upper face of thesecond protection insulating film 111 is placed at substantially thesame level as the upper face of the capacitor lower electrode 110A.

[0096] A capacitor dielectric film 112A of an oxide dielectric film suchas a ferroelectric film and a high dielectric film with a thickness of10 nm through 200 nm is formed on the second protection insulating film111 so as to have a lower face in contact with the capacitor lowerelectrode 110A. The capacitor dielectric film 112A is in contact withthe capacitor lower electrode 110A and has a plane shape larger thanthat of the capacitor lower electrode 110A. The oxide dielectric film isnot specified in its kind, and may be a ferroelectric film having abismuth-layered perovskite structure such as SrBi₂(Ta_(1-x)Nb_(x))O₉, ora film of lead zirconate titanate, strontium barium titanate, tantalumpentaoxide or the like.

[0097] A capacitor upper electrode 113A of a platinum film with athickness of, for example, approximately 50 nm is formed on thecapacitor dielectric film 112A, and the capacitor dielectric film 112Aand the capacitor upper electrode 113A are covered with a thirdprotection insulating film not shown.

[0098] In the semiconductor device of Embodiment 3, a multi-layerstructure including the first upper oxygen barrier layer 114A and thesecond upper oxygen barrier layer 115A is formed between the oxygenbarrier layer 108A and the capacitor lower electrode 110A. Therefore,the diffusion of the oxygen atoms can be more effectively prevented thanin the semiconductor device of Embodiment 2. As a result, the oxidationof the plug 107 can be further definitely prevented.

[0099] Embodiment 4

[0100] In Embodiment 4 of the invention, a method for fabricating thesemiconductor device of Embodiment 2 will be described. A method forfabricating the semiconductor device of Embodiment 1 or 3 is basicallythe same as the fabrication method for the semiconductor device ofEmbodiment 2 and hence is omitted.

[0101] First, as shown in FIG. 2A, a gate electrode 102 of a transistoris formed on a semiconductor substrate 100 by a known method, and a pairof impurity diffusion layers 101 serving as the source and the drain ofthe transistor are formed in regions of the semiconductor substrate 100on both sides of the gate electrode 102.

[0102] Next, after forming a first protection insulating film 103 of,for example, a TEOS-O₃ film on the semiconductor substrate 100 so as tocover the transistor, the first protection insulating film 103 isplanarized by CMP. Thereafter, the first protection insulating film 103is selectively etched, so as to form a plug opening 104 by exposing oneof the pair of impurity diffusion layers 101.

[0103] Then, as shown in FIG. 2B, a barrier metal 105 composed of alower titanium film (with a thickness of 30 nm) and an upper titaniumnitride film (with a thickness of 50 nm) and a tungsten film 106 (with athickness of 600 nm) are successively deposited on the first protectioninsulating film 103 so as to fill the plug opening 104. Thereafter,portions of the barrier metal 105 and the tungsten film 106 exposedoutside the plug opening 104 are removed by the CMP, thereby forming aplug 107 as shown in FIG. 2C.

[0104] Next, as shown in FIG. 2C, an oxygen barrier layer 108 with athickness of 20 nm through 200 nm of a composite nitride that is amixture or an alloy of a first nitride having a conducting property anda second nitride having an insulating property is deposited on the firstprotection insulating film 103. The first nitride may be a nitride of atleast one of titanium, tantalum, cobalt, copper and gallium, and thesecond nitride may be a nitride of at least one of aluminum, silicon,chromium, iron, zirconium and hafnium.

[0105] Subsequently, an upper oxygen barrier layer 109 with a thicknessof, for example, 100 nm of a metal that has a conducting property evenwhen oxidized is deposited on the oxygen barrier layer 108. The metalthat has a conducting property even when oxidized may be at least one ofiridium, ruthenium, rhenium, osmium, rhodium, platinum and gold. Insteadof the metal that has a conducting property even when oxidized, theupper oxygen barrier layer 109 may be made from a metal oxide having aconducting property including at least one of an iridium oxide, aruthenium oxide, a rhenium oxide, an osmium oxide and a rhodium oxide.

[0106] Then, a first platinum film 110 with a thickness of, for example,approximately 50 nm is deposited on the upper oxygen barrier layer 109by sputtering.

[0107] Next, as shown in FIG. 3A, the first platinum film 110, the upperoxygen barrier layer 109 and the oxygen barrier layer 108 aresuccessively patterned, thereby forming a capacitor lower electrode 110Afrom the first platinum film 110 and forming a patterned upper oxygenbarrier layer 109A and a patterned oxygen barrier layer 108A.

[0108] Then, a second protection insulating film 111 of, for example, aTEOS-O₃ film with a thickness of 400 nm is formed on the firstprotection insulating film 103 so as to cover the capacitor lowerelectrode 110A, the patterned upper oxygen barrier layer 109A and thepatterned oxygen barrier layer 108A.

[0109] Subsequently, as shown in FIG. 3B, the second protectioninsulating film 111 is planarized by the CMP so as to place the upperface of the second protection insulating film 111 at substantially thesame level as the upper face of the capacitor lower electrode 110A.Then, an oxide dielectric film 112 of a ferroelectric film or a highdielectric film with a thickness of 10 nm through 200 nm is deposited onthe planarized second protection insulating film 111. The oxidedielectric film is not specified in its kind, and may be a ferroelectricfilm having a bismuth-layered perovskite structure such asSrBi₂(Ta_(1-x)Nb_(x))O₉, or a film of lead zirconate titanate, strontiumbarium titanate, tantalum pentaoxide or the like. Also, the method forforming the oxide dielectric film 112 may be metal organic decomposition(MOD), metal organic chemical vapor deposition (MOCVD), the sputteringor the like.

[0110] Next, a second platinum film 113 with a thickness of, forexample, approximately 50 nm is deposited on the oxide dielectric film112 by the sputtering.

[0111] Then, as shown in FIG. 3C, the second platinum film 113 and theoxide dielectric film 112 are successively patterned, so as to form acapacitor upper electrode 113A from the second platinum film 113 and acapacitor dielectric film 112A from the oxide dielectric film 112 bothin a plane shape larger than that of the upper oxygen barrier layer 109Aand the oxygen barrier layer 108A. In this case, since the secondplatinum film 113 and the oxide dielectric film 112 are formed on theplanarized second protection insulating film 111, there does not arise aproblem of etching residue after patterning the second platinum film 113and the oxide dielectric film 112.

[0112] Thereafter, the resultant semiconductor substrate 100 issubjected to annealing carried out in an oxygen atmosphere at atemperature of 600 through 800 for 10 through 60 minutes, therebycrystallizing the capacitor dielectric film 112A.

[0113] During the annealing for crystallization, oxygen atoms includedin the oxygen atmosphere, which are to diffuse through the upper oxygenbarrier layer 109A and the oxygen barrier layer 108A to reach the plug107, are obstructed not only by the upper oxygen barrier layer 109A madefrom the metal that has a conducting property even when oxidized or themetal oxide having a conducting property but also by the oxygen barrierlayer 108A made from the composite nitride that is a mixture or an alloyof the first nitride having a conducting property and the second nitridehaving an insulating property. As a result, the oxygen atoms minimallyreach the plug 107.

[0114] The mechanism of the upper oxygen barrier layer 109A forpreventing diffusion of the oxygen atoms is described in Embodiment 2,and the mechanism of the oxygen barrier layer 108A for preventing thediffusion of the oxygen atoms is described in Embodiment 1.

[0115] Also, during the annealing for the crystallization, the oxygenatoms included in the oxygen atmosphere pass through the secondprotection insulating film 111 before reaching the upper oxygen barrierlayer 109A and the oxygen barrier layer 108A. Therefore, the number ofoxygen atoms that can reach the upper oxygen barrier layer 109A and theoxygen barrier layer 108A is reduced.

[0116] Furthermore, since the plane shape of the capacitor dielectricfilm 112A is larger than that of the capacitor lower electrode 110A, theoxygen atoms included in the oxygen atmosphere migrate by a longdistance within the second protection insulating film 111 beforereaching the upper oxygen barrier layer 109A and the oxygen barrierlayer 108A. Therefore, the number of oxygen atoms that can reach theupper oxygen barrier layer 109A and the oxygen barrier layer 108A isfurther reduced.

[0117] Accordingly, the number of oxygen atoms that can reach the plug107 can be thus largely reduced, so as to definitely prevent oxidationof the plug 107.

[0118] Now, an electric characteristic test carried out on aconventional semiconductor device and the semiconductor device ofEmbodiment 2 will be described.

[0119] First, by using the structure of the conventional semiconductordevice or the semiconductor device of Embodiment 2, contact chains areprepared each by serially connecting 1000 semiconductor devices throughplugs by sharing impurity diffusion layers and capacitor lowerelectrodes (so that an (n−1)th transistor can share an impuritydiffusion layer with an nth transistor and the nth transistor can sharea capacitor lower electrode with an (n+1)th transistor) with thediameters of the plugs varied from 0.22 ìm to 0.30 ìm by 0.01 ìm in therespective constant chains. The annealing for crystallizing thecapacitor dielectric films is carried out by keeping the semiconductorsubstrates at a substrate temperature of 700 for 1 hour in an oxygenatmosphere.

[0120]FIG. 4 shows a failure occurrence probability measured in a testin which every contact chain including the serially connected 1000semiconductor devices is decided to be faulty when its resistanceobtained in applying a predetermined voltage to the both ends thereofexceeds a predetermined value. It is understood from FIG. 4 that thefailure occurrence probability is remarkably low in using thesemiconductor device of Embodiment 2. For example, when the plug has adiameter of 0.24 ìm, the failure occurrence probability is 0% in thecontact chain using the semiconductor device of Embodiment 2 while thefailure occurrence probability is 98% in the contact chain using theconventional semiconductor device. Thus, the characteristic can beremarkably improved.

[0121]FIG. 5 shows contact resistance corresponding to resistance ofeach plug calculated by dividing, by 1000, resistance obtained byapplying a predetermined voltage to the both ends of each contact chainincluding the serially connected 1000 semiconductor devices. When theplug has a diameter of 0.24 ìm, the contact resistance of thesemiconductor device of Embodiment 2 is 400 Ù while the contactresistance is 4 kÙ or more in the conventional semiconductor device. Inthis manner, according to Embodiment 2, contact resistance applicable toan actual device can be obtained even when high temperature annealing iscarried out in an oxygen atmosphere.

What is claimed is:
 1. A method for fabricating a semiconductor devicecomprising the steps of: forming an impurity diffusion layer serving asa source or a drain of a transistor in a semiconductor substrate;forming a first protection insulating film covering said transistor;burying, in said first protection insulating film, a plug having a lowerend electrically connected to said impurity diffusion layer of saidtransistor; forming, on said first protection insulating film, an oxygenbarrier layer having a lower face connected to an upper end of saidplug; forming a capacitor lower electrode on said oxygen barrier layer;forming, on said first protection insulating film, a second protectioninsulating film covering said oxygen barrier layer and said capacitorlower electrode, and planarizing said second protection insulating film,whereby placing an upper face of said second protection insulating filmat substantially the same level as an upper face of said capacitor lowerelectrode; forming a capacitor dielectric film having a plane shapelarger than a plane shape of said capacitor lower electrode bydepositing an oxide dielectric film on said capacitor lower electrodeand said second protection insulating film and patterning said oxidedielectric film; and forming a capacitor upper electrode on saidcapacitor dielectric film.
 2. The method for fabricating a semiconductordevice of claim 1, wherein said oxygen barrier layer is made from acomposite nitride that is a mixture or an alloy of a first nitridehaving a conducting property and a second nitride having an insulatingproperty.
 3. The method for fabricating a semiconductor device of claim1, further comprising, between the step of forming said oxygen barrierlayer and the step of forming said capacitor lower electrode, a step offorming an upper oxygen barrier layer made from a metal that has aconducting property when it is oxidized.
 4. The method for fabricating asemiconductor device of claim 1, further comprising, between the step offorming said oxygen barrier layer and the step of forming said capacitorlower electrode, a step of forming an upper oxygen barrier layer madefrom a metal oxide having a conducting property.